We propose to explore the question of why in many antibody responses, only one or a few B-cell clones are activated and their products expressed for years in preference to the antibody molecules produced by other clones. Possible explanations for the preferential expression of a limited group of antibody molecules can be based on immunologic unresponsiveness, the strength of the immunogenic stimulus, and the affinities of B-cell receptors. These possible explanations will be tested by conducting detailed structural and physico-chemical studies on the antibody molecules which are expressed in preference to all others of the same specificity under varying conditions of immunization. These studies will be facilitated by the proximity and the availability of a large colony of genetically characterized rabbits, many of which are inbreeding.